1. Field of the Invention
This invention relates to a digital tracker of aircraft to provide range and critical values of time to collision in a collision avoidance system for at least 16 targets substantially simultaneously including at least one pair of co-range targets.
2. Description of the Prior Art
Collision avoidance systems have been proposed as means of preventing collisions by vehicles both of the maritime vessel type and of the airborne type. Cooperative systems of the asynchronous type include interrogator-transponder techniques in which interrogation signals or probes are transmitted from one vehicle on a random basis. Vehicles in the vicinity receiving such probes respond with signals of various indicia to provide to the interrogation vessel information relating to both range and information identifying the vessel and its position. In airborne systems such information may include the relative or actual altitude of the aircraft.
An existing collision avoidance system known by the acronym SECANT (Separation Control of Aircraft by NonSynchronous Techniques) employs probes identified by any one of a plurality of frequencies and replies using different frequencies of the same band but arranged into a predetermined correspondence to a particular probe frequency. Special correlation techniques aboard any vehicle separate the true reply to its probes from other signals which may be replies to probes generally identified as "fruit" from remote vessels. Such a system is described in U.S. Pat. Nos. 3,755,811 issued Aug. 28, 1973, and 3,803,609, issued Apr. 9, 1974, based on the inventions of Jack Breckman as well as U.S. Pat. No. 3,803,604 issued Apr. 9, 1974, to Bernard Case.
In such collision avoidance systems, the ability to track more than one target at a time depends upon expanding the circuit components of the system to accommodate each additional target to be tracked. Such an arrangement for multiple target tracking is not only complex but costly due to the additional components required.
In addition, the ability to track two targets that are converging to a point where their response signals would appear to the system to be a single target is a problem that can generate spurious bits which interfere with the proper operation of the tracker.